Electricity transmitted through power lines destined for commercial, industrial and residential use can involve hundreds of thousands of volts and high currents. To measure the voltage, the utility worker must contact a high voltage line, which presents a significant risk to the worker. Indeed, mere proximity to a high voltage line is extremely dangerous. Nonetheless, in installing, servicing and repairing power lines, measurements of the voltage and phasing parameters such as the phase sequence or rotation must be made.
The circumstances and equipment used for measuring voltage and phase sequence of transmission lines vary considerably. For example, prior art phasing voltmeters with long interconnect cords are customarily used for measuring the voltage on individual lines of a multi-phase transmission power lines. This corded phasing volt meter helps to prevent two lines that are not in phase from being connected inadvertently. When other parameters of the AC voltage, such as the phase sequence or rotation of the voltage on the conductors, are to be determined, a completely different instrument or a whole set of instruments and additional equipment costing many thousands of dollars must be used.
However, if a prior art type phasing voltmeter such as the one described in U.S. Pat. No. 6,459,252 were improved as described herein, it could be used to measure voltage parameters such as leading phase, lagging phase, voltage phase sequence and phase rotation when a voltage measurement is taken and thereby minimize the number of times the worker approaches or contacts a high voltage line to reduce the risk to the worker and eliminate additional equipment, thus avoiding injuries and saving thousands of dollars in expenses.
There are four well-known, distinctly different voltage measurements that may be made on a three-phase power line. The present voltmeter is capable of all four and adds a fifth voltage measurement, heretofore never mentioned. This fifth measurement accurately determines the magnitude of a source of measurement inaccuracy among high voltage phasing meters when making the four standard measurements mentioned above.
Historically, high voltage measurements made using a corded type high voltage phasing meter have been plagued with inaccuracies stemming from stray capacitive charging currents. At high voltages, stray charging currents emanate from the surface of every component of the measuring device including the cord itself. The capacitive current is related to the capacitive reactance, Xc, which can range from several thousand ohms up to millions of ohms, depending on the location of the meter and its cable with respect to ground or grounded objects. Under extreme conditions, such as when a long series cable of a corded phasing volt meter is lying directly on the ground between two pad-mounted transformers, the value of the capacitive reactance can be very low resulting in a capacitive current that can equal or exceed the measured current.
While at least one prior art phasing voltmeter is able to reconcile the stray AC capacitive charging currents to provide accurate voltage indications under all conditions, even it fails to recognize, analyze and extract the valuable information contained in the charging current. There remains a need for an improved phasing voltmeter that has the ability to measure and analyze the stray AC capacitive charging current in order to provide additional voltage-related information to the user based on a single contact with the high voltage line.